298 lines
9.3 KiB
Markdown
298 lines
9.3 KiB
Markdown
---
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name: capture-screen
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description: Programmatic screenshot capture on macOS. Find window IDs with Swift CGWindowListCopyWindowInfo, control application windows via AppleScript (zoom, scroll, select), and capture with screencapture. Use when automating screenshots, capturing application windows for documentation, or building multi-shot visual workflows.
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---
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# Capture Screen
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Programmatic screenshot capture on macOS: find windows, control views, capture images.
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## Quick Start
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```bash
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# Find Excel window ID
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swift scripts/get_window_id.swift Excel
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# Capture that window (replace 12345 with actual WID)
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screencapture -x -l 12345 output.png
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```
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## Overview
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Three-step workflow:
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```
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1. Find Window → Swift CGWindowListCopyWindowInfo → get numeric Window ID
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2. Control View → AppleScript (osascript) → zoom, scroll, select
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3. Capture → screencapture -l <WID> → PNG/JPEG output
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```
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## Step 1: Get Window ID (Swift)
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Use Swift with CoreGraphics to enumerate windows. This is the **only reliable method** on macOS.
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### Quick inline execution
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```bash
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swift -e '
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import CoreGraphics
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let keyword = "Excel"
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let list = CGWindowListCopyWindowInfo(.optionOnScreenOnly, kCGNullWindowID) as? [[String: Any]] ?? []
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for w in list {
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let owner = w[kCGWindowOwnerName as String] as? String ?? ""
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let name = w[kCGWindowName as String] as? String ?? ""
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let wid = w[kCGWindowNumber as String] as? Int ?? 0
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if owner.localizedCaseInsensitiveContains(keyword) || name.localizedCaseInsensitiveContains(keyword) {
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print("WID=\(wid) | App=\(owner) | Title=\(name)")
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}
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}
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'
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```
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### Using the bundled script
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```bash
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swift scripts/get_window_id.swift Excel
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swift scripts/get_window_id.swift Chrome
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swift scripts/get_window_id.swift # List all windows
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```
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Output format: `WID=12345 | App=Microsoft Excel | Title=workbook.xlsx`
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Parse the WID number for use with `screencapture -l`.
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## Step 2: Control Window (AppleScript)
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Verified commands for controlling application windows before capture.
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### Microsoft Excel (full AppleScript support)
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```bash
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# Activate (bring to front)
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osascript -e 'tell application "Microsoft Excel" to activate'
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# Set zoom level (percentage)
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osascript -e 'tell application "Microsoft Excel"
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set zoom of active window to 120
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end tell'
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# Scroll to specific row
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osascript -e 'tell application "Microsoft Excel"
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set scroll row of active window to 45
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end tell'
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# Scroll to specific column
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osascript -e 'tell application "Microsoft Excel"
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set scroll column of active window to 3
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end tell'
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# Select a cell range
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osascript -e 'tell application "Microsoft Excel"
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select range "A1" of active sheet
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end tell'
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# Select a specific sheet
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osascript -e 'tell application "Microsoft Excel"
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activate object sheet "DCF" of active workbook
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end tell'
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# Open a file
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osascript -e 'tell application "Microsoft Excel"
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open POSIX file "/path/to/file.xlsx"
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end tell'
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```
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### Any application (basic control)
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```bash
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# Activate any app
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osascript -e 'tell application "Google Chrome" to activate'
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# Bring specific window to front (by index)
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osascript -e 'tell application "System Events"
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tell process "Google Chrome"
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perform action "AXRaise" of window 1
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end tell
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end tell'
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```
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### Timing and Timeout
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Always add `sleep 1` after AppleScript commands before capturing, to allow UI rendering to complete.
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**IMPORTANT**: `osascript` hangs indefinitely if the target application is not running or not responding. Always wrap with `timeout`:
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```bash
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timeout 5 osascript -e 'tell application "Microsoft Excel" to activate'
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```
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## Step 3: Capture (screencapture)
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```bash
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# Capture specific window by ID
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screencapture -l <WID> output.png
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# Silent capture (no camera shutter sound)
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screencapture -x -l <WID> output.png
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# Capture as JPEG
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screencapture -l <WID> -t jpg output.jpg
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# Capture with delay (seconds)
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screencapture -l <WID> -T 2 output.png
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# Capture a screen region (interactive)
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screencapture -R x,y,width,height output.png
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```
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### Retina displays
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On Retina Macs, `screencapture` outputs 2x resolution by default (e.g., a 2032x1238 window produces a 4064x2476 PNG). This is normal. To get 1x resolution, resize after capture:
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```bash
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sips --resampleWidth 2032 output.png --out output_1x.png
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```
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### Verify capture
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```bash
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# Check file was created and has content
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ls -la output.png
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file output.png # Should show "PNG image data, ..."
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```
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## Multi-Shot Workflow
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Complete example: capture multiple sections of an Excel workbook.
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```bash
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# 1. Open file and activate Excel
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osascript -e 'tell application "Microsoft Excel"
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open POSIX file "/path/to/model.xlsx"
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activate
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end tell'
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sleep 2
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# 2. Set up view
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osascript -e 'tell application "Microsoft Excel"
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set zoom of active window to 130
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activate object sheet "Summary" of active workbook
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end tell'
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sleep 1
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# 3. Get window ID
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# IMPORTANT: Always re-fetch before capturing. CGWindowID is invalidated
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# when an app restarts or a window is closed and reopened.
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WID=$(swift -e '
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import CoreGraphics
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let list = CGWindowListCopyWindowInfo(.optionOnScreenOnly, kCGNullWindowID) as? [[String: Any]] ?? []
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for w in list {
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let owner = w[kCGWindowOwnerName as String] as? String ?? ""
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let wid = w[kCGWindowNumber as String] as? Int ?? 0
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if owner == "Microsoft Excel" { print(wid); break }
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}
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')
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echo "Window ID: $WID"
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# 4. Capture Section A (top of sheet)
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osascript -e 'tell application "Microsoft Excel"
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set scroll row of active window to 1
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end tell'
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sleep 1
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screencapture -x -l $WID section_a.png
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# 5. Capture Section B (further down)
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osascript -e 'tell application "Microsoft Excel"
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set scroll row of active window to 45
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end tell'
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sleep 1
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screencapture -x -l $WID section_b.png
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# 6. Switch sheet and capture
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osascript -e 'tell application "Microsoft Excel"
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activate object sheet "DCF" of active workbook
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set scroll row of active window to 1
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end tell'
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sleep 1
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screencapture -x -l $WID dcf_overview.png
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```
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## Failed Approaches (DO NOT USE)
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These methods were tested and confirmed to fail on macOS:
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| Method | Error | Why It Fails |
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|--------|-------|-------------|
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| `System Events` → `id of window` | Error -1728 | System Events cannot access window IDs in the format screencapture needs |
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| Python `import Quartz` (PyObjC) | `ModuleNotFoundError` | PyObjC not installed in system Python; don't attempt to install it — use Swift instead |
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| `osascript` window id | Wrong format | Returns AppleScript window index, not CGWindowID needed by `screencapture -l` |
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## Permission Troubleshooting
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`swift scripts/get_window_id.swift` reads on-screen windows via CoreGraphics, so it needs Screen Recording permission on macOS.
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Use this order:
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1. Confirm trigger
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2. Confirm target identity
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3. Add/enable exact app in Settings
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If the command fails with `ERROR: Failed to enumerate windows`, do this:
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```bash
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open "x-apple.systempreferences:com.apple.preference.security?Privacy_ScreenCapture"
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```
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Or print the same checklist directly from the script:
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```bash
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swift scripts/get_window_id.swift --permission-hint screen
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swift scripts/get_window_id.swift --permission-hint microphone
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```
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Then:
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1. In Privacy & Security → Screen Recording, enable the target app.
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2. If your app is missing from the list:
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- Ensure you granted permission to the real app bundle (not `swift` / terminal helpers).
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- For CLI tools, build/run as a packaged `.app` during permission verification.
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- Click `+` and add the `.app` manually from `/Applications`.
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3. Re-run the command after restarting the app.
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4. If this is a CLI workflow, also check whether the launcher is a helper binary:
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- In most cases the entry shown in TCC is the helper process (`swift`, `Terminal`, `iTerm`, etc.), not the business app.
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- Permission still works after helper-level grant, but it is not ideal for final UX.
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For mic-access-related prompts, use the same pattern with the microphone pane:
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```bash
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open "x-apple.systempreferences:com.apple.preference.security?Privacy_Microphone"
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```
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The same rule still applies: the system can only show permissions for a concrete `.app` bundle. If the request is made by a helper binary, the settings list can be misleading or empty for your product app.
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### Quick Check Template
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```text
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1) Error: permission denied
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2) Open target pane
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3) Verify identity shown by OS = identity you granted
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4) If not matched, use the script-reported candidate identities and grant the launcher process
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5) Reopen/restart and verify
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```
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For production apps, avoid requesting permissions via `swift`/`python` entry points; always route permission checks in the packaged app process so users only see one target.
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If you maintain another macOS permission-related flow, reuse this standardized triage template:
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- [permission-triage-template.md](references/permission-triage-template.md)
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## Supported Applications
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| Application | Window ID | AppleScript Control | Notes |
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|------------|-----------|-------------------|-------|
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| Microsoft Excel | Swift | Full (zoom, scroll, select, activate sheet) | Best supported |
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| Google Chrome | Swift | Basic (activate, window management) | No scroll/zoom via AppleScript |
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| Any macOS app | Swift | Basic (activate via `tell application`) | screencapture works universally |
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AppleScript control depth varies by application. Excel has the richest AppleScript dictionary. For apps with limited AppleScript, use keyboard simulation via `System Events` as a fallback.
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